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Title: Calculating excited states of condensed matter
Author: Webster, Ross
ISNI:       0000 0004 5367 5685
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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The accurate and reliable prediction of the optical properties of extended systems is a powerful tool for the development of nanostructured optical devices, such as photovoltaic materials. One major challenge faced by excited-state electronic structure methods is the treatment of the two-particle electron-hole interaction which is a key component of optical response properties such as optical absorption and charge transport. In this work we first study the effect of choice of basis set on both ground-state and first-order response properties, extending the molecular studies of Rappoport et al. into periodic and crystalline systems. From the conclusions of this basis set study, we employ a series of polarisation-optimised basis sets to calculate the optical gaps of the alkali halide series AX (where A = Li, Na, K, Rb and X = F, Cl, Br). Secondly, we assess the applicability of a range of methods (HF, LDA, PBE and B3LYP) for excited state calculations of extended systems, and examine the dependence of optical properties on the contribution of Hartree-Fock exchange (cHF ) to the linear-response TD-B3LYP kernel. We find that TD-B3LYP underestimates both the fundamental gap (Eg ) and optical gap (Eo ) in comparison to experimental work, and we show that the optical gap can be reproduced at least quantitatively with cHF ≃ 0.3. The last chapter of this work focusses on the calculation of electronic and optical properties of single-walled carbon nanotubes (SWCNTs). We study the effect of geometry relaxation, curvature, basis set and method on one-electron and linear-response properties, comparing our findings to other theoretical and experimental works. Our study shows that B3LYP is able to accurately and reliably calculate the ground-state electronic properties of a large range of nanotubes ( < 2-46 Å in diameter). Through our geometry optimisations and studies on nanotube diameter, we observe the curvature effect on the qualitative and quantitative properties of the van Hove singularities and Density of States of SWCNTs. We find that TD-B3LYP produces values of Eo that are in good agreement with experimental measurements and GW-BSE calculations, for both metallic and semiconducting nanotubes, however the estimated exciton binding energies for our calculations, Eb (where Eb = Eg - Eo) are in poor agreement with previous theoretical calculations. This is likely due to our comparison of two-particle (Eo ) values and roughly-estimated values of Eg from single-particle eigenvalues.
Supervisor: Harrison, Nicholas Sponsor: Science and Technology Facilities Council ; Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral